The present invention relates to an electric discharge machining control method and an electric discharge machining control device in which an electrode and a workpiece are placed in a machining liquid, a voltage is applied between the electrode and the workpiece, and the workpiece is machined with the generated electric discharge.
In an electric discharge machine, an electrode and a workpiece are placed in a machining liquid, a voltage is applied between the electrode and the workpiece, and the workpiece is machined with the generated electric discharge. A gap distance control system for adjusting the distance between the electrode and the workpiece is provided in the electric discharge machine that maintains a stable machining state. FIG. 16 shows the configuration of a machining control system including the conventional gap distance control system. This configuration is described, for example, on p88 to p90 of xe2x80x9cMechanism of Electric Discharge Machining and How to Make Full Use of Itxe2x80x9d (published by Gijutsu Hyoron-sha). As shown in this figure, designated at the reference numeral 101 is an electric discharge machining process, at 102 a machining-state detecting section, at 104 a reference-value setting section, at 105 an error-signal computing section, at 106 a control variable computing section, at 107 a machining-trajectory setting section, at 108 a rotational condition setting section, at 109 an electrode driving unit, at 110 a machining-pulse condition setting section, and at 111 a machining power unit. Designated at the reference sign y is a quantity of state in the electric discharge machining process 101, at ym a detected value of the quantity of state y detected by the machining-state detecting section 102, at r a reference value of a machining state set in the reference-value setting section 104, at e an error signal computed from the detected value ym and the reference value r by the error-signal computing section 105, at Rv a machining-trajectory instruction value set in the machining-trajectory setting section 107, at Rr a rotation instruction value set in the rotational condition setting section 108, at Up is a control variable computed from the error signal e and the machining-trajectory instruction value Rv by the control variable computing section 106, at Mp an electrode moving quantity operated according to the control variable Up and the rotation instruction value Rr by the electrode driving unit 109, at Rs a machining-pulse condition instruction value set in the machining-pulse condition setting section 110, and at Ms a machining pulse quantity operated according to the instruction value Rs by the machining power unit 111. It should be noted that the machining-trajectory instruction value Rv and the control variable Up are a vector variable corresponding to XYZ axes, the rotation instruction value Rr is a vector variable corresponding to a rotating speed and direction around a C axis. The electrode driving unit 109 consists of an XYZ-axial driving unit and a C-axial rotating unit, and controls a gap distance in the XYZ-axial direction while rotating the electrode in the C-axial direction according to the electrode moving quantity Mp. In addition, the machining-pulse condition instruction value Rs consists of an open voltage, a peak current, a pulse-ON time, and a pulse-OFF time, or the like.
FIG. 17 shows the operation of the conventional gap distance control system. A gap distance control algorithm is generally executed through software-processing by a microcomputer, and this figure shows a k-th processing. The processing in Step S201 is performed in the machining-state detecting section 102 and in this step a machining state of the electric discharge machining process as, for instance, an average gap voltage ym (k) is detected. The processing in Step S401 is performed in the error-signal computing section 105 and in this step an error signal e (k) from the reference value r and the detected value ym (k) of the average gap voltage is computed. The processing in Step S1701 is performed in the control variable computing section 106 and in this step a control variable Up (k) from the machining-trajectory instruction value Rv set in the machining-trajectory setting section 107 and the error signal e (k) is calculated. Further, the control variable computing section 106 gives the control variable Up (k) to the electrode driving unit 109. Here, Kp is a proportional gain and Ki is an integral gain. The electrode is then moved in such a manner that the detected value ym (k) coincides with the reference value by using the well-known PI compensation (proportional and integral compensation).
FIG. 18 shows a power spectrum of a detected value indicating a machining-state in the conventional gap distance control system. Namely, this figure shows the power spectrum P of an average gap voltage ym (k) detected by the machining-state detecting section 102 when machining is executed by the electric discharge machine having the conventional gap distance control system. As can be seen in this figure, the power spectrum shows peaks at frequencies f1, f2, and f3. These peaks correspond to rotational frequencies of the electrode and the harmonic of the frequencies. The peaks in the power spectrum are derived from eccentric of the electrode to be rotated as well as from fluctuations in electric characteristics at the feeding brush section. FIG. 19 shows a power spectrum of a machining-state detected value in another electric discharge machine using the conventional gap distance control system. Namely, this figure shows a power spectrum P of an average gap voltage ym (k) detected by the machining-state detecting section 102 when machining is executed by another electric discharge machine having the conventional gap distance control system. When compared with FIG. 18, a new peak appears at the frequency f4. This is a resonance frequency of the mechanical system of the XYZ-axial driving unit. Therefore, if the mechanical system has many resonance frequencies then peaks corresponding to each resonance frequency appear in the power spectrum. As described above, the presence of peaks in the power spectrum of the detected average gap voltage indicates the fact that there exists disturbance to the gap distance control system at the frequency corresponding to each peak. A stable machining state cannot be maintained at such frequencies where a peak is present.
As described above, for machining using a rotated electrode, eccentric of the electrode varies a gap distance, which disturbs the gap distance control system and deteriorates the machining speed. In addition, rotation of the electrode fluctates the electric characteristics at feeding brush sections, because machining current is fed from a machining power unit to the electrode through the feeding brush sections, which disturbs the gap distance control system and deteriorates the machining speed. Further, when there is mechanical resonance in the electrode driving unit, a control variable to the driving unit and an actual movement of an electrode are different, which results into an inappropriate control of the gap distance and deteriorates the machining speed.
A first electric discharge machining control method according to the present invention comprises the steps of detecting a machining state, filtering a detected value indicating the machining state with a notching frequency, computing an error signal from an output value by means of filtering and a set value of a machining state, computing a control variable for controlling movement of an electrode from the error signal and a set movement value of the electrode, and moving the electrode in a specific direction and at the same time rotating the electrode vertical to the opposite surface to a workpiece according to the control variable. With those operations, disturbance to a gap distance control system can be suppressed. Therefore, machining speed can be improved by maintaining a stable machining state.
In a second electric discharge machining control method according to the present invention, a notching frequency is adjusted according to a rotational frequency of an electrode or to a mechanical resonance frequency of a driving system for moving and rotating the electrode. With those operations, disturbance to a gap distance control system due to eccentric of an electrode as well as to fluctuations in electric characteristics can be suppressed. Therefore, machining speed can be improved by maintaining a stable machining state.
A third electric discharge machining control method according to the present invention comprises the steps of detecting a machining state, computing an error signal from a detected value indicating the machining state and a set value of a machining state, computing a control variable for controlling movement of an electrode from the error signal and a set movement value of the electrode, computing a correction for reducing an eccentricity of the electrode, compensating the control variable according to the correction, and moving the electrode in a specific direction and at the same time rotating the electrode vertical to the opposite surface to a workpiece according to the compensated control variable. With those operations, disturbance to a gap distance control system due to eccentric of an electrode as well as to fluctuations in electric characteristics can be suppressed. Therefore, machining speed can be improved by maintaining a stable machining state.
A fourth electric discharge machining control device according to the present invention comprises a machining-state detecting section for detecting a machining state, a notch filter section for filtering a detected value in the machining-state detecting section, an error-signal computing section for computing an error signal from an output value from the notch filter section and a set value of a machining state, a control variable computing section for computing a control variable for controlling movement of an electrode from the error signal and a set movement value of the electrode, and an electrode driving section for moving the electrode in a specific direction and at the same time rotating the electrode vertical to the opposite surface to a workpiece according to the control variable outputted from the control variable computing section. With those operations, disturbance to a gap distance control system can be suppressed. Therefore, machining speed can be improved by maintaining a stable machining state.
A fifth electric discharge machining control device according to the present invention further comprises a notching-frequency self adjustment section for adjusting a notching frequency according to a rotational frequency of an electrode or to a mechanical resonance frequency of a driving system for moving and rotating the electrode. With those operations, disturbance to a gap distance control system due to eccentric of an electrode as well as to fluctuations in electric characteristics at feeding brush sections can be suppressed. Therefore, machining speed can be improved by maintaining a stable machining state.
A sixth electric discharge machining control device according to the present invention comprises a machining-state detecting section for detecting a machining state, an error-signal computing section for computing an error signal from a detected value in the machining-state detecting section and a set value of a machining state, a control variable computing section for computing a control variable for controlling movement of an electrode from the error signal and a set movement value of the electrode, a correction computing section for computing a correction for reducing an eccentricity of the electrode, a control variable compensating section for compensating the control variable from the control variable computing section according to the correction from the correction computing section, and an electrode driving section for moving the electrode in a specific direction and at the same time rotating the electrode vertical to the opposite surface to a workpiece according to the compensated control variable. With those operations, disturbance to a gap distance control system due to eccentric of an electrode as well as to fluctuations in electric characteristics at feeding brush sections can be suppressed. Therefore, machining speed can be improved by maintaining a stable machining state.